The present invention relates generally to the field of power distribution equipment. More particularly, the invention relates to loadbreaking connectors for distribution equipment. Still more particularly, the invention relates to separable loadbreaking bushing and elbow connectors used to connect distribution conductors to transformers and other equipment.
Separable connectors are typically employed to interconnect sources of energy, such as electrical distribution network conductors, to localized distribution components, such as transformers. These connectors, for example, typically include a bushing insert, which is mounted in the bushing well of the transformer, and an elbow connector which is releasably connected to the bushing insert. In this application the bushing insert and bushing well combination are replaced with a one piece bushing. The bushing electrically connects to a transformer winding and the elbow is connected to a distribution conductor of the network circuit feeding the transformer. When the elbow is interconnected to the bushing, the transformer is thus interconnected into the distribution network and thereby energized. Likewise, if the elbow is removed, the transformer is disconnected from the distribution network and the transformer is de-energized.
To carry electric current through the separable connectors and into the transformer from the distribution conductor, each of the separable components include a conductive member which serves as the current carrying path. The conductive member in the elbow includes an elongated metallic rod which forms a probe connector. The conductive member in the bushing includes a female contact, which receives the probe as the elbow is pushed over the bushing to make a connection. The elbow is structured in a general L-shape such that the distribution conductor is received in one arm of the elbow and interconnected to the probe therein, and the probe is retained in and extends through the other arm of the elbow and disposed generally at a right angle to the conductor. The probe is protected by an insulative shroud which is circumferentially disposed about the probe such that there is an annular space between the probe and the inner surface of the shroud. When the elbow is interconnected to the bushing, a portion of the bushing is received within this annular space, and the probe is received within the female contact in the bushing.
The female contact is disposed inside the bushing within a contact tube. The female contact includes a cylindrical probe receiving portion into which the probe of the elbow is engaged when the elbow is placed onto the bushing. This receiving portion typically includes a tulip contact which is configured with a series of longitudinal slots through the end thereof. The material between the slots forms petals which are inwardly biased in a radial direction such that the receiving end of the contact, prior to the reception of the probe, has a diameter smaller than the diameter of the probe. The petals of the contact are actuable radially outward upon reception of the probe therein. The female contact is commonly referred to as a tulip contact, because the arrangement looks like the flower of a tulip plant. The elasticity of the tulip contact petals create an inward spring force to cause the contact to grip the probe. To permit the tulip contact to expand radially outward within the contact tube to receive the probe, a gap, or clearance annulus, is provided between the outer surface of the tulip contact and the inner diameter of the contact tube.
The distribution conductor, and thus the elbow probe, is commonly energized during normal use and may be energized both during installation of the elbow over the bushing and when the elbow is removed from the bushing. As a result, the materials used in the bushing and elbow must be capable of withstanding the extreme temperature and pressures that are generated during electrical arcing which can occur as the live, or energized, probe comes into contact with or is disengaged from the tulip contact.
During the interconnection of the elbow and bushing while the conductor is energized, as the probe comes into the proximity of the tulip contact, the voltage gradient between the live probe and the non-energized tulip contact increases. This gradient is measured in terms of voltage difference between the line voltage of the elbow and the potential of the bushing before the elbow is placed on the bushing, and the distance between high and low voltage components. The voltage between the elbow and bushing contacts may be as high as a phase to phase voltage of 36,600 volts, for example, and the line to ground maximum is 21.1 KV. When the probe is first inserted into the bushing, the differential voltage between the probe and tulip contact is supported by the dielectric strength of the air gap between the probe and the conducting components within the bushing. Arcing occurs when the dielectric strength of the weakest resistance path between the probe and tulip contact is less than the voltage differential between the probe and tulip contact. This path commonly includes both the air gap between the tulip contact and probe, as well as portions of the surface and structure of the elbow and bushing components. As the probe and tulip contact come closer together, the air gap component of the weakest resistance path decreases, thereby increasing the likelihood of an arc between the probe and contact along the weakest resistance path. The types and dimensions of the materials used in the elbow and bushing are selected to ensure that an arc-over condition should not prematurely occur along the surface of the elbow or bushing. This is accomplished by selecting internal components having a high dielectric resistance. This, combined with the dielectric resistance of the air gap between the probe and tulip contact as the elbow is slipped over the bushing, tends to prevent the incidence of arcing until the probe is within the contact tube containing the tulip contact. However, once the probe is in the immediate vicinity of the tulip contact, the dielectric strength of the air gap and/or adjacent component structures and surfaces may be exceeded, and an arc will then form between the probe and tulip contact. This arc between the probe and tulip contact will conduct currents which may be as high as the available fault current. However, in normal operation, the current is limited to 200 amps, per ANSI/IEEE standards. This arc will follow the path of least resistance between the probe and tulip contact, such path commonly including the interior surface of the contact tube and the outer surface of the probe follower. As the probe is moved further towards the tulip contact, the probe and tulip contact make physical contact and the arc will be extinguished as steady-state contact is achieved. In a similar manner, as the elbow is pulled off of the bushing while the components are in an energized state, an arc will again form between the probe and tulip contact as they separate.
The generation of arcs during the interconnection and disconnection of the elbow and bushing can lead to bushing and elbow degradation and failure. The energy and heat created during an arc can melt and burn the adjacent surface of the contact tube and carbonize the surface of the interior structure of the bushing and elbow causing them to lose their insulative qualities. More specifically, the occurrence of carbonization can create carbonized, and thus conductive, leakage paths along the surface of the bushing and elbow components, which will lead to further mechanical and electrical degradation of the bushing. Additionally, the gasses given off as the arc burns or melts the elbow and bushing components creates high pressures in the vicinity of the probe-tulip contact interface. Because this area is confined within the contact tube, with the probe blocking the opening of the tube, the gas pressure that is generated acts to impart an outward force on the probe which tends to repel the probe coming into contact with the tulip contact. This force, in turn, requires the installer to apply greater force to the back of the elbow to push the probe into contact with the tulip contact. If the installer inserts the elbow too slowly, or with insufficient force, the duration of the contact to probe arc can be greatly increased. The longer the arc is permitted to exist, the greater the chance of damage to the elbow and bushing components and the greater the gas pressure generated and the force needed to install the elbow on the bushing. Thus, both the elbow and bushing must be designed to minimize arcing.
To address the problems presented by arcing, the bushing typically includes one or more seals to seal out moisture and dirt which would otherwise enhance the surface conductivity of the bushing components and prematurely initiate arcing which will interfere with the probe-to-tulip contact engagement. The bushing may also include an ablative insert that is positioned in the location where the arc typically forms. This insert ablates, or vaporizes, when an arc contacts it. Upon ablation, the insert produces a gas which serves to help extinguish the arc. Additionally, many prior art devices employ additional features such as sliding, spring-loaded contacts, and magnetic inserts to help force the tulip contact and probe into engagement. Prior art devices which employ ablative inserts commonly include a tubular member molded from the ablative material which also forms a pilot for aligning the probe with the tulip contact. For example, FIG. 5 of U.S. Pat. No. 4,863,392, discloses an ablative tubular insert 230 which is disposed in the bushing 200 immediately in front of the tulip contact 224. The insert 230 terminates prior to engagement with the tulip contact 224, leaving a gap between these elements. Likewise, U.S. Pat. No. 3,957,332 discloses a quench tube 21 which terminates just prior to contacting the end of contact 17. This same basic configuration is disclosed in U.S. Pat. Nos. 4,186,985 and 4,773,872.
Despite the prior advancements in the art, the arcs created in present-day connectors can still induce the formation of carbonized paths and burning and melting of the elbow and bushing components. It has been found that some arcs will roll over the end of the tulip contact into the clearance annulus and destructively melt and vaporize the contact tube adjacent the clearance annulus. In conventional connectors, the arc can avoid engagement with the ablative insert for a not insubstantial distance and period of time, permitting the arc to substantially damage the bushing components.